The invention relates to an A/D conversion device having an input intended to receive an analog input signal, and an output intended to supply a digital output signal, comprising:
an amplifier having an output and an input intended to receive the analog input signal of the device, provided with regulating means, controlled by a digital control signal, for adjusting the value of a voltage supplied by the output of the amplifier, PA1 an A/D converter having an analog input connected to the output of the amplifier and an output intended to supply the digital output signal of the device. PA1 a comparison module, activated when the calibration signal is active, comprising a comparator intended to supply, at an output, a signal which is representative of at least one comparison between the digital output signal and a predetermined binary word, PA1 a control module having an input connected to the output of the comparison module and being activated when the calibration signal is active, intended to supply the control signal, whose value depends on the value of a signal received at the input of the control module, and PA1 storage means in which the value of the control signal is stored when the calibration signal is inactive. PA1 the second current source comprises N current sources which are parallel arranged between the second power supply terminal and a node which is common for all the sources, the value I.sub.i of the nominal current supplied by the current source of rank i (for i=1 to N) being equal to I0/2.sup.(i-1) in which I0 is a predetermined value, PA1 the control signal is a binary word of N+1 bits, of which a correction bit and its logic inverse control the first and second switches, respectively, each residual bit, when being in an active state, controlling the conduction of one of the N current sources constituting the second current source. PA1 the calibration signal being active during at least N times a comparison cycle, in the course of which an analog signal at the output of the amplifier is converted into a digital signal and then compared with the predetermined binary word, the comparison module is provided with a register intended to store, in the course of a control cycle whose duration is at least equal to that of a comparison cycle, the value taken by the output signal of the comparator in the course of at least one of the previous control cycles, PA1 the control module is provided with means for placing, in the course of the first control cycle, the correction bit of the control signal at the active state if the value of the signal which it receives at its input indicates that the digital output signal has a higher value than the predetermined binary word, and at the inactive level in the opposite case, and those bits which control the conduction of the source of rank 1 systematically at the active state, PA1 the control module is provided with means for systematically placing at the active state, in the course of any subsequent control cycle, the bit of the rank subsequent to that of the previous bit whose state has been modified in the course of the previous cycle, the previous bit being unchanged if the value of the signal which it receives at its input is identical to that of the signal received at the end of the previous control cycle, and inversed in the opposite case, the process being repeated until the calibration signal resumes the inactive state. PA1 the calibration signal being active during at least N.P comparison cycles, each control cycle comprises P comparison cycles, PA1 the comparison module is provided with a register intended to store the values taken by the output signal of the comparator at the end of each one of the P successive comparisons carried out in the course of a same control cycle, and with a weighting module intended to supply, at an output, a signal whose value is representative of an average of said values, said output constituting the output of the comparison module. PA1 a first A/D conversion device having an input and an output respectively intended to receive the analog input signal of the device and supply a signal of a digital kind, referred to as most significant word, constituting the most significant part of the digital output signal of the circuit, PA1 a D/A converter intended to receive the most significant word and to convert it into an output signal of an analog kind, referred to as converted signal, PA1 a second AID conversion device having an analog input intended to receive the difference between the converted signal and the input signal of the circuit, and an output intended to supply a signal of a digital kind, referred to as least significant word constituting the least significant part of the digital output signal of the circuit,
Such a device is described in U.S. Pat. No. 5,313,207. This device is used within an A/D conversion circuit having a two-step structure in which a primitive analog signal is converted into a digital signal by a first A/D converter which supplies, at the output, a digital signal referred to as most significant word which constitutes an approximation of the digital value of the analog input signal and thus forms the most significant part of the digital output signal of the conversion circuit. The most significant word weight is converted into an analog signal by a D/A converter which supplies, at the output, a signal referred to as converted signal which corresponds to the transposition to an analog value of the approximate digital value supplied by the first A/D converter. Here, this converted signal takes the form of a current which acts on a device as described in the opening paragraph, in which the amplifier receives the primitive analog signal at the input. In this device, the control signal is actually constituted by the most significant word, the operation consisting of adjusting the value of the output voltage of the amplifier via the regulating means incorporating the analog conversion of said word. The amplifier thus supplies, at the output, an analog signal referred to as residual signal which is representative of a quantization error due to the approximation effected by the first A/D converter, which residual signal is converted within the device by a second A/D converter into a digital signal referred to as least significant word which thus forms the least significant part of the digital output signal of the conversion circuit. The amplifier in the known A/D conversion device can introduce a DC component, referred to as offset voltage, in the output signal which it supplies. This has detrimental consequences for the precision of the A/D conversion performed by the second converter. Indeed, this offset voltage artificially modifies the value of the signal which occurs at the input of said converter and may thus cause discontinuities in the development of the value of the output signal of the device as a function of the development of the value of the primitive analog signal, which is not acceptable.